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Linux Weather Forecast

Jon Corbet's Linux Weather Forecast

LWN executive editor Jonathan Corbet updates his report on current and future conditions of Linux kernel development.

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The 3.10 Merge Window Closes

Linus Torvalds made a Mother's Day gift to the world in the form of the 3.10-rc1 kernel prepatch. With this release, the merge window for the 3.10 development cycle has closed, so we know which features to expect this time around.

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The Cracking of

As has recently been announced on the main page, the main server (known as “hera”) was recently compromised by an unknown intruder. This person was able to gain “root” access, meaning they had the full run of the system. Speaking as just one of many members of the kernel development community, I can say that this episode is disturbing and embarrassing. But I can also say that there is no need to worry about the integrity of the kernel source or of any other software hosted on the systems. is, of course, the home for the Linux kernel. Many other projects live there as well. On the face of it, that would make a tempting target for an attack. What self-respecting cracker wouldn’t want an opportunity to place some special code into the Linux kernel? Such code would, over time, find its way into millions of machines worldwide. The injection of backdoors or other malware is a concern for any software maintainer - open source or otherwise - but it turns out that we are well protected against that sort of attack. If kernel developers worked by shipping simple files of source code around, they might well be vulnerable to malware added by an intruder. But that is not how kernel development is done. The code for the kernel (and for many other projects) is managed with the “git” source code management system. And git does not allow the code to be modified by third parties without people knowing about it.

It’s worth taking a moment to look at how that works. A cryptographic “hashing function” is a mathematical formula which boils the contents of a file down to a small number. “Small” is relative; git’s hash function produces 160-bit numbers, which are quite big by...

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What's Up With ARM

Over the course of the last month or so, numerous people have asked me for my opinion on what’s going on with the ARM architecture in Linux. It seems time to broadcast those thoughts more widely.

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2.6.38: Making Things Just Work

Linus Torvalds announced the release of the 2.6.38 kernel on March 14. Like its predecessors, 2.6.38 incorporates a lot of work - over 9,500 patches from over 1,100 developers. There are a number of useful changes, including some important scalability improvements, but, in my mind, the most interesting theme behind this kernel is that of making advanced features Just Work. “Pages” are the units of memory as understood by the processor’s memory management unit. Since the beginning, Linux has used 4096-byte pages on most architectures - the smallest size that the MMU understands. Contemporary processors can handle a number of page sizes simultaneously, though, with 2MB or 4MB often being the next largest size available. Larger pages require less overhead to manage, but the real value to their use is that they greatly increase the amount of memory which can be covered by the processor’s translation lookaside buffer (TLB). The TLB, which caches virtual-to-physical address translations, is a severely limited resource on most systems. But it is important; a TLB miss can cost many hundreds of processor cycles even if the destination page is fully resident in memory. A 2MB page requires one TLB entry; the same memory, in 4096-byte pages, needs 512 TLB entries. So using huge pages can save a lot of TLB misses, leading to significant performance increases, especially in virtualized situations. Linux has supported the use of huge pages for years, but...

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